1. Field of the Invention
This invention relates to a projection exposure apparatus for projecting a pattern image onto a substrate to be exposed by the use of a projection optical system, and in particular to an exposure apparatus having a position detecting device for effecting position detection when a pattern on a pattern substrate such as a reticle is transferred onto a substrate to be exposed.
2. Related Background Art
An exposure apparatus of this type generally has, as a position detecting device, a device for effecting the detection of the position of a projection optical system for pattern transfer in a direction orthogonal to the optic axis and a device for effecting the detection of the position of the projection optical system in the direction of the optic axis. The device for effecting the former position detection, as disclosed in U.S. Pat. No. 4,677,301 has one position detecting device using a sensitizing wavelength and another position detecting device using a non-sensitizing wavelength differing from the sensitizing wavelength of a wafer. A fiducial mark or a reference mark provided on a stage has been detected with the aid of the two position detecting devices by running the stage, and by monitoring the position coordinates of the stage, and the relative distance of the detection positions of the two position detecting devices (the base line) has been measured from the amount of movement of the stage, the fiducial mark of the stage has been aligned with the mark of a reticle by said one position detecting device, and a wafer mark has been detected by said another position detecting device, and the stage has been moved by said relative distance (the base line), whereby alignment of the reticle mark and the wafer mark has been indirectly effected relative to said one position detecting device.
However, in this apparatus of the prior art, only the measurement of said relative distance (the base line measurement) is effected when the reticle is placed onto a reticle holder, and in the sequence of wafer alignment and exposure, confidence has been put in the aforementioned relative distance measured during the reticle setting, whereafter the reticle has not been observed.
More particularly, in the reticle alignment sequence, a reticle is placed onto the reticle holder and the reticle is aligned in directions, X, Y and .theta., whereafter measurement of the magnification is effected and the reticle is focused to the fiducial mark of the stage, and the relative distance of the two position detecting devices is measured. Also, in the wafer exposing sequence, a wafer is placed onto a wafer holder and the wafer is global-aligned in directions X, Y and .theta.. Thereafter, alignment of each chip is effected by the use of a non-sensitizing wavelength, and the wafer is exposed after the correction of the relative distance.
In the prior art as described above, the process of observing the reticle and the fiducial mark of the stage at the same time through the same optical system is carried out only when the reticle is placed onto the reticle holder, and this has led to the problem that the detection of the position of the wafer has been subject to an offset due to a variation with time in the optical system and the accuracy of the position detection has suffered.
On the other hand, effecting the measurement of the relative distance of the two position detecting devices (used for the measurement of the magnification or the measurement of the base line)(which is the detection of the position in a plane orthogonal to the exposure optic axis) or the measurement of the in-focus state (which is the detection of the position in the direction of the exposure optic axis) results in a reduced throughput and therefore has not been practiced in the past.